What does optocoupler mean?

What does optocoupler mean?

A photocoupler is a semiconductor device that converts an electronic signal into an optical signal and then back to an electronic signal. When current moves to the input side of the photocoupler (Figure 4-8), the optical signal is output by the light-emitting diode. The optical sensor on the output side detects it and the current moves at the same time.

The signs and phenomena of the electric coupler are shown in Figure 4-9.

Figure 4-8: Operation principle of electric coupler

Figure 4-9: Electric coupler

Figure 4-10 shows a circuit diagram used by Ichikawa to detect excessive current circuits. When the current exceeding the original level is input to the primary side of the coupler, the secondary side is grounded and 0V is input to the microcomputer. Please note that due to the difference in current supply between the primary and secondary sides of the coupler, noise can be filtered and interference with signal transmission can be avoided.

Figure 4-10: Circuit using an electric coupler

Structural characteristics of photocouplers

The main structure of the photocoupler is to assemble the light-emitting device and the light-receiving device in a sealed tube shell, and then use the pin of the light-emitting device as the input end, and the pin of the light receiver as the output end. When an electrical signal is added to the input end, the light-emitting device emits light. In this way, the light-receiving device generates a photocurrent after being illuminated due to the photosensitivity effect and outputs it from the output end. Thus, the transmission of electrical signals using "light" as the medium is realized, and the input and output ends of the device are electrically insulated. In this way, a new type of semiconductor electronic device that transmits signals through light in the middle is formed. The packaging forms of the photocoupler are generally tubular, dual in-line and optical fiber connection. Figure 1 is a circuit diagram of three series of photocouplers.

The main features of optocoupler are as follows:

●Insulation between input and output terminals, the insulation resistance is generally greater than 10 10Ω, the withstand voltage can generally exceed 1kV, and some can even reach more than 10kV.

●Due to the unidirectional nature of "light" transmission, there will be no feedback when the signal is transmitted unidirectionally from the light source to the light receiver, and its output signal will not affect the input end.

●Since the light-emitting device (GaAs infrared diode) is an impedance current driven device, and noise is a high internal resistance micro-current voltage signal, the common mode rejection ratio of the photoelectric coupling device is very large, so the photoelectric coupling device can effectively suppress interference and eliminate noise.

●Easy to coordinate with logic circuits.

●Fast response speed. The time constant of optocoupler devices is usually in microseconds or even nanoseconds.

●No contact, long life, small size, impact resistance.

Current status of development of optocouplers

Although the market for Japanese photocouplers is not very large, it is increasing at an annual rate of 40%. The main reason is that 20 to 30 or even more photocouplers are used in each program controller. Now, photocouplers have shown a clear trend towards large capacity and high speed. The photocouplers produced in the United States and Japan are mainly composed of infrared light-emitting diodes and photosensitive device tubes, which account for about 60% of all photocouplers produced in the United States and Japan. Because this type of device not only has high current transmission efficiency (generally 7 to 30%), but also has a relatively fast response speed (2 to 5μs), it can meet the requirements of most applications. The three high-speed photocouplers made by Yokogawa Electric Corporation of Japan using GaAsP infrared light-emitting diodes as input terminals and PIN photodiodes as receiving terminals have an insulation voltage of more than 3000 volts. Among them, the response speed of the 5082-43610 ultra-high-speed digital photocoupler and the 5082-4361 high common-mode suppression photocoupler can reach 10Mb/s, and their current transmission efficiency is as high as more than 60%. The 4N25, 4N26, and 4N27 photocouplers produced by Motorola in the United States are triode output type photocouplers [2]. This type of photocoupler has very high input and output insulation performance, a frequency response of up to 300kHz, and a switching time of only a few microseconds.

Among the high-speed photocouplers produced by NEC, the PS2101 photocoupler is a general-purpose four-pin flat component that combines an AlAs-GaAs infrared light-emitting diode and a silicon phototransistor and packages it in a volume of 4×4.4×2 cubic millimeters, with a response speed of 10μs. The PS2041 and PS2042 photocouplers are six-pin package components that integrate AlAs-GaAs light-emitting diodes and phototransistors on the same substrate, with a size of 7.08×7.6×3.5 cubic millimeters and a response time of 0.3μs.

In recent years, relevant domestic units have invested a lot of manpower and material resources to research and develop various optocoupler devices. Such as Shanghai Semiconductor Device Factory No. 8, Shanghai Radio Factory No. 17, etc. In order to meet market needs, Chongqing Optoelectronic Technology Research Institute has developed a thick film integrated dual-channel high-speed high-gain electric coupler consisting of a high-speed response light-emitting device and a logic output type optical receiving amplifier. The input end of this optocoupler consists of two GaAIAs side-emitting tubes, and its output end consists of two Si-PIN photodetectors and two high-speed high-gain linear amplifier circuits. Figure 2 shows its principle circuit diagram. In addition, Chongqing Optoelectronic Technology Research Institute has also developed high-speed high-voltage optocouplers, GG2150I RF signal optocouplers, GG2060I high-voltage pulse measurement optocouplers, GH1204U high-voltage optical transmission optocouplers, and GH1201Y and GOHQ-I optocouplers.

Typical Applications of Optocouplers

1. Used as a solid state relay

Optocoupler is a novel optoelectronic device that combines a light-emitting diode and a phototransistor. It uses optical signals to transmit information, so that the input of the circuit is completely isolated from the electrical circuit. This information transmission method is incomparable to all general solutions that use transformers and relays for isolation to transmit signals. Since the optocoupler has the characteristics of unidirectional information transmission, wide bandwidth, small parasitic feedback, strong noise elimination ability, and good anti-electromagnetic interference performance, it has been more and more widely used in both digital circuits and analog circuits.

The use of photocouplers as solid relays has the advantages of small size, close coupling, low driving power, fast action speed, and wide operating temperature range. Figure 3 shows an actual circuit diagram of a photocoupler used as a solid relay. The left half of the circuit can be used to convert the input electrical signal Vi into a light signal emitted by the light-emitting diode in the photocoupler; while the right half of the circuit restores the light signal to an electrical signal through the photosensitive transistor in the photocoupler, so this is a very good electro-optical and photoelectric combined conversion device. The current transmission ratio of the photocoupler used in the figure is 20%, the withstand voltage is 150V, and the driving current is between 8 and 20mA. In actual use, since it does not have the actual contacts commonly seen in general electromagnetic relays, there is no poor contact and arcing, and it will not cause malfunctions due to external force or mechanical shock. Therefore, its performance is relatively reliable and its operation is very stable.

2. Application of photocouplers in telephone security devices

In order to prevent the telephone line from being stolen by parallel machines or the telephone from being stolen for calls, a simple and practical telephone security circuit can be designed using a photocoupler. The actual isolation circuit is shown in Figure 4. In the figure, VD1~VD4 form a polarity conversion circuit. Since it is not necessary to distinguish the polarity of the feedback voltage of the telephone line when connecting this protector to the telephone line, the use of this protector can greatly facilitate the installation.

Normally, in the on-hook state, the 48V or 60V feed voltage in the line (the feed voltage is different for different switch models) is rectified by VD1~VD4, limited by R2, and stabilized by Zener diode VD, so that the light-emitting tube at the input end of the IC lights up, causing the light-receiving device at the output end of the IC to turn on and the transistor V to be cut off, the relay K does not work, the contact K-1 in the control circuit is disconnected, R1 is not connected to the telephone line, and the circuit is in a normal monitoring state. At the same time, K-2 is also in the disconnected position, and the electronic alarm buzzer does not sound.

Once the telephone is stolen or the phone is stolen, the feeder voltage in the line drops to 6-10V. Since the 6-100V voltage cannot cause VD to break down after being rectified by VD1-VD4, the light-emitting tube at the input end of the IC will not light up, which will turn the output end from on to off and make the resistance infinite, and then the base of V will obtain the base current through the resistor R3 and turn on. The relay is pulled in to close the K-1 contact, and the resistor R1 is connected to the telephone line in parallel, so that the voltage in the line further drops to below 10V, so that the telephone cannot be stolen by the parallel machine. At the same time, the telephone cannot work due to abnormal voltage, thus preventing the telephone line from being stolen or the telephone from being stolen. At the same time, K-2 is closed, and the power supply of the electronic buzzer is turned on to make it sound an alarm. And the two light-emitting diodes on the shell of the electronic buzzer are lit and glow. If the telephone owner makes a call, just press SB to disconnect it, that is, cut off the power supply of the protector.

The IC of the telephone security device can be selected from photoelectric couplers of 4N25, 4N36, etc. The relay K can be selected from relays of JRX-12F, etc., with a working voltage of 6 volts DC. In order to reduce the size of the casing, the battery can be selected from 6 volt laminated batteries. SB is a normally closed push button switch. B can be selected from FMQ-27, FMQ-35 electronic buzzers, and the remaining parameters can be selected according to the markings in the figure.

3. Use optocouplers instead of audio transformers

In linear circuits, audio transformers are often used for coupling between two-stage amplifiers. The disadvantage of this coupling is that some power will be lost in the transformer iron chip and may cause some distortion. If optocouplers are used instead of audio transformers, these disadvantages can be overcome.

At present, there are many forms of application circuits that use optocouplers to replace audio transformers. Figure 5 shows a more practical one. When the input signal Vi is amplified by the front stage of transistors BG1 and BG2, it drives the LED on the left side of the optocoupler to emit light, and is completely absorbed by the photosensitive tube on the right and converted into an electrical signal. This signal is amplified by the back stage circuit BG3, and the emitter of the tube outputs an undistorted amplified signal V0 after passing through the capacitor C3. Since the circuit completely isolates the front and rear amplifiers, the interference that may be caused by the ground loop is eliminated. At the same time, since the circuit also has a noise reduction function, signal distortion is avoided. The total gain of the entire circuit is expected to reach more than 20dB, and the bandwidth is about 120kHz. In addition, optocouplers can also be used to form logic circuits such as AND gates, OR gates, and NAND gates, as well as isolated solid-state switch circuits, dual voltage regulator circuits, choppers, and differential amplifier circuits.